Reserves for Loss and Loss Expenses (Narrative) (Details) - USD ($) $ in Thousands	9 Months Ended		12 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Liability for Claims and Claims Adjustment Expense [Line Items]
Net of loss reserve discounts	$ 16,787	$ 12,085
Decrease in estimates for claims	30,609	45,813
Adjustment expense	30,600	$ 42,000	$ 42,000
Insurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Adjustment expense	62,000		38,000
Reinsurance
Liability for Claims and Claims Adjustment Expense [Line Items]
Adjustment expense	$ 31,000		$ 4,000